Pixels, or “picture elements,” are the basic light- or color-detection and display elements that form a digital image. Typical digital video and imaging systems use a collection of detector pixels to capture a two-dimensional image field at a capture end (such as a camera) and another corresponding collection of display pixels to display the corresponding two-dimensional image at a display end (such as a monitor). In digital imaging systems, an array of light-sensitive pixels, each including a light sensor or detector, respond to an intensity of incident light at each pixel location, providing an electrical output representative of the incident light. The output of an imager can be referred to as an image.
Motion or video cameras repeat the process described above, but permit a time-sequence to be captured, for example at regular intervals, so that the captured images can be replayed to recreate a dynamic scene or sequence.
Most film and digital pixel imagers include wavelength-specific sensors or detectors. The chemical composition of the film or the design of the digital pixels and associated filters determines the range of wavelengths of light to which the film or pixels respond. Practically, a detector or imager has a frequency response that is optimized to provide images of light in the range of wavelengths the imager is designed for. The most common examples are sensitive to visible light (e.g., red, green, blue, and combinations thereof). Visible light corresponds to the range of wavelengths of electromagnetic radiation to which our eyes are sensitive, and is generally in the range of 400 to 750 nanometers (nm).
Special film and digital pixel imagers are designed for low-light operation to provide night vision capability for military, security, or other special applications in which an illumination source is not available to cause a visible light image. Low-light or night vision imagers rely on detecting and imaging frequencies below (wavelengths longer than) the visible (red) wavelengths, and are sometimes called infra-red (IR) detectors. IR detection is more suited for picking up heat emissions from objects such as a person's body or a vehicle. IR radiation itself can be roughly divided into sub-spectra including the near-infra-red (NIR) having wavelengths between about 750 to 1100 nm, short-wave-infra-red (SWIR) having wavelengths between about 1100 and 2500 nm, medium-wave-infra-red (MWIR) having wavelengths between about 2500 and 8000 nm, and long-wave-infra-red (LWIR) having wavelengths between about 8000 and 12000 nm. These ranges are defined somewhat arbitrarily, and are given merely for simplifying the following discussion, and those skilled in the art will appreciate the generality of the discussion as it relates to the bands of wavelengths of the electromagnetic spectrum.
Present visible light imaging cameras have used silicon devices made with CID, CCD, or CMOS APS architectures. The low cost and efficient collection of photons from 400-750 nm wavelengths has enabled silicon devices. Present low-light or night vision IR imagers are usually less sensitive than would be desired, lack color definition, and have limited frequency response. Also, low-light imagers can be more costly, noisy, and require greater circuit resources than visible light imagers to achieve useful gains in low-signal conditions. Furthermore, IR sensors are larger than would be desired for compact portable applications because most IR sensitive materials must be cooled significantly to achieve good performance. Most long-wavelengths tend to have higher dark currents at a particular temperature. IR image sensors offer one example of sensors with higher dark currents as compared to the previously mentioned visible light detectors.
Generally, the signal output of an imaging pixel can be increased by increasing the integration time, and long integration times are generally used for low-light-level operation. However, this approach is ultimately limited by dark current leakage that is integrated along with the photocurrent. One of the problems associated with high dark current photodetectors is the limit on well capacity. A significant portion of the charge collection well can be filled just with the dark current, which reduces the over-all dynamic range by cutting in to the maximum signal that can be collected. As the dark current fills the device well, there is less room for the photoelectrons and the dark current can saturate the well. One method to reduce dark current is to cool the imaging device. Other methods to reduce dark current include using “pinning implants” in photogate or photodiode pixels and post processing algorithms which may sample the imaging device with a closed shutter to subtract dark current offset. Often some of these techniques may be used in conjunction to remove dark current offset and improve image quality.
Many detectors also suffer from a high background signal. This is especially the case in IR image sensors where the desired image object does not have a large thermal gradient with respect to the background. The high background signal produces a current offset that is similar to dark current.
In summary, present imaging sensors and pixels do not sufficiently remove dark current or background offset current for certain applications and improved pixel architecture is needed for such detectors.